Undercut commutator



sept. 7, 1965 T. J. BQYER 3,205,387

UNDERCUT COMMUTATOR Filed Dec. 20, 1962 2 Sheets-Sheet 2 INVENTOR 7790/14/75 J.' BOYER ,SYM ,gm UMZZWM'A? United States Patent e 3,205,387 a UNDERCUT COMMUTATOR Thomas J. Boyer, Livonia, Mich., assignor to Ford Motor Company, Dearborn, Mich., a corporation of Delaware Filed Dec. 20, 1962, Ser. No. 246,099 3 Claims. (Cl. S10-233) This invention relates to a method of undercutting the insulators of a commutator employed on dynamoelectric machines and to the commutator produced thereby.

In a dynamoelectric machine, it is desirable to use a narrow undercut at the commutator insulator segments to preserve the maximum amount of commutator bar surface in contact with the brushes of the machine. A narrow undercut is also effective in reducing the magnitude of brush noise, It is very difficult, however, to utilize a narrow undecut and still remove all of the insulator segment at the undercut. The reason for this difficulty is that commutator bars are usually not `perfectly formed, and as a result the commutator segments may not be properly positioned angularly within the commutator. Also, the commutator bars and insulator segments are often poorly indexed relative to one another, even in premium commutators. This means that the circumferential dimension between the insulator segments may not be uniform. In mass production techniques, therefore, it is difficult to employ machines that utilize a narow undercut and will at the same time remove all of the insulator segments at the under'cuts. This results in poor commutation, poor performance, poor durability and poor brush life, and it also results in brush noise.

The present invention is directed to a method of obviating the above-mentioned difficulties and to the commutato'r formed thereby. The method of the invention entails undercutting the insulator segment in such a manner as to produce a dovetail slot. The slot is narrow at the commutator surface and widens at the deepest `portion of the cut toward the center of the commutator. This provides the benefit of a narrow slot at the surface of the commutator so that the commutator has low brush noise characteristics and has maximum commutator bar Vsurface in Contact with the brushes of the dynamoelectric machine. It also provides for the wide cut necessary to free the slot of all the insulating material.

a The dovetail slot `may be achieved by means of a circular milling cutter that is inclined at a slight angle with respect to the insulator segment. A first cut is then taken along the length of the insulator segment. A circular milling cutter is then inclined at a similar angle in the opposite direction with respect to the insulator segment and a second cut is taken. This Produces a dovetail slot at each insulator segment.

In the preferred manner of performing the method of the invention, the dovetail slots may be formed by the use of two circular milling cutters that are `mounted on a common shaft or arbor. One circular milling cutter will make all of the cuts at Vthe insulator segments inclined in one direction and the other circular milling cutter will make all of the cuts inclined in the opposite direction. After the two circular milling cutters have made a pair of cuts into a set of insulator segments, the commutator is rotated about its axis to bring the next set of insulator segments into proper cutting relationship with these circular milling cutters. Mounting the two circular mill"- ing cutters on a common shaft will assist greatly in holding the exact location of the second cut at each of the insulator segments relative to the first cut at each of the insulator segments.

An object of the invention is the provision of a method for undercutting the insulator segments on commutators that will remove all of the insulator at the undercut area and that will leave a maximum amount of commutator bar surface in contact with the brushes of the dynamoelectric machine.

Another object of the invention is to provide a method for undercutting the insulator segments of commutators that forms an undercut slot in a commutator at each insulator segment that is narrow at the surface of the commutator and that iswide at the portion of the slot positioned nearest the center of the commutator.

`A further object of the invention is the provision of a commutator that provides a maximum amount of commutator bar surface in contact with the brushes of a dynamoelectric machine and that simultaneously has all of the insulating'material of the insulator segments removed in the area of the undercut slots of the insulator segments. e

Other objects and attendant advantages of the invention will become more apparent as the specification is considered in connection with the attached drawings in which;

FIGURE 1 is a perspective view of the commutator of the present invention; l n

FIGURE 2 is an enlarged sectional view of a portion ofl the commutator prior to undercutting showing adjacent conductive bars or segments separated by an insulator segment; a

FIGURE 3 is a View similar to FIGURE 2 and showing how the commutator is undercut, and

FIGURE 4 is a partial schematic view of a means for 'carrying out the methods of the invention.

Referring now to kFIGURE 1, there is shown the cornmutator 10 of the present invention that is formed of a plurality of `commutator bars or conductive segments 11 that are separated by a plurality of insulator segments 12. As conventional in commutator structures, the conductive segments 11 are formed of copper bars while the insulator segments 12 are formed of a mica base insulating material. The commutator is formed so that the alternating conductive bars or segments 11 and insulator ksegments `12 form a cylindrical array in which the insulator segments 12 extend radially inwardly from the surface 13 of the commutator toward the center of the cylindrical array. The commutator 10 is undercut at each of the insulator segments 12 to form a dovetail slot 14t'hat extends through the length of the commutator that is adapted to contact an electrical contact brush. This dovetail 4slot is narrow at the surface 13 of the commutator and is wider at the bottom thereof toward the center of the commutator 10. This type of slot or undercut `has the advantages pointed out in the introductory Aportion of the specication. It provides a maximum amount of commutator segments or bars in contact with the electrical brushes of the dynamoelectric machine and it insures that all of the insulating material is removed in the area of the undercut.

Referring now to FIGURES 2 and k3, the method of undercuttingithe insulator segments 12 of the commutator 10 and forming the dovetail slots 14 will now be described. The method comprises making a r'st cut into the commutator at the insulator segment 13 that is `inclined in one direction with respect to the insulator segment. This first cut is diected into the conductive segment 11 positioned o'n one side of the insulator segment. A second cut is then made at the insulator segment 12 in a direction that is inclined to the insulator segment in a direction opposite to the first cut. This second cut is directed into the conductive segment 11 positioned on the other side of the insulator segment 12. It can be appreciated that these two cuts form the dovetail slot 14 at the insulator segment and that the narrow portion of the dovetail slot at the surface 13 Iof the commutator spans or is wider than the mica segment at this position. Al-

though the method is not so limited, it is preferred that the two inclined cuts be made of the same width (wider than the insulator segment 12) and be in the same position on the surface 13 of the commutator 10 as shown in FIGURES 2 and 3. It is to be understood that the rst and second cuts described above extend through the length of the commutator that may come into contact with the electrical brushes of the dynamoelectric machine.

A practical means for making the dovetail slots 14 in the commutator and for carrying out the method of the invention is shown in FIGURE 4. A first circular milling cutter 21 and a second circular milling cutter 22 are mounted on a common rotatable shaft 23. This shaft is adapted for movement downwardly, as shown in FIG- URE 4, to make the iirst and second cuts described above and for movement in a direction perpendicular to the plane of the drawings so that the cuts are made through a desired length of the commutator. The circular milling cutter 21 makes the rst cut, as described above, and the circular milling cutter 22 makes the second cut, as described above, since the circular milling cutter 22 is inclined at an angle with respect to the conductive segments undergoing cutting opposite to the angle of the circular milling cutter 21. In practice, the commutator assembly is rotated through one conductive segment 11 after the circular milling cutters 21 and 22 each make a cut. It can be seen that the dovetail slot 14 will be completed at each of the insulator segments 12 when the circular milling cutter 21 has made a first cut at each of the insulator segments 12 and the second circular milling cutter 22 has made a second cut at each of the insulator segments 12.

It can be readily appreciated by an inspection of FIG- URE 4 that the circular milling cutters 21 and 22 are rotatable about an axis that is substantially normal to the direction of the radially extending insulator segment 12A that is positioned between the circular milling cutters 21 and 22 and the insulator segments 12B and 12C that are being cut by these cutters.

Considered in a broader aspect, it can be appreciated that the rst cut at an insulator segment 12 is made by a circular milling cutter that is rotatable about an axis that is substantially normal to the adjacent insulator segment positioned on one side of the insulator segment undergoing cutting. The second cut is made by a milling cutter that is rotatable about an axis that is substantially normal to the adjacent insulator segment positioned on the other side of the insulator segment undergoing cutting. This can be appreciated by an inspection of FIGURE 4 and by considering that the commutator 10 is rotated or indexed in a counterclockwise direction.

As viewed in FIGURE 4, the insulator segment 12C is being cut by the circular milling cutter 22. The circular milling cutter 22 is rotatable about an axis that is substantially normal to the insulator segment 12A positioned to the left of the insulator segment 12C. After 'this cutting operation is performed, the second cut in the insulator segment 12C will be made by the circular milling cutter 21 after the commutator assembly has been rotated or indexed counterclockwise to bring the insulator segment 12C under the circular milling cutter 21. In this position, the circular milling cutter 21 is rotatable about an axis that is substantially normal to the insulator segment 12D that is now positioned between the circular milling cutters 21 and 22. The insulator segment 12D is the insulator segment adjacent the insulator segment 12C and positioned on the other side thereof from the insulator segment 12A. From this description of the method of cutting the dovetail slots 14 at the insulator segments 12 of the commutator 10 to undercut the commutator, it can be seen that the commutator assembly may remain stationary and that both circular milling cutters 21 and 22 may be revolved about the commutator assembly 10 to make the cuts in the directions described above. It can also be appreciated that aA single milling cutter could be employed to cut the dovetail slots.

The advantage of using two circular milling cutters on a common shaft is that this assembly will assist greatly in holding the exact location of the second cut relative to the rst cut.

The present invention thus provides a method for undercutting the insulator segments of a commutator in which the undercut is narrow at the surface to preserve the maximum amount of conductive bars or segments in contact with the brushes and is wider as the undercut deepens to insure that all the insulator in the area of the undercut is removed.

It will be understood that the invention is not to be limited to the exact construction shown and described, but that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.

I claim:

1. A commutator for a dynamoelectric machine comprising a first conductive segment, a second conductive segment, an insulator separating said first and said second conductive segments, said commutator including a dovetail slot positioned in the surface of said commutator at said insulator with the narrow portion of said dovetail slot positioned at the surface of said commutator and being of sutiicient width that none of the insulator segment is present at the surface of said commutator.

2. A commutator for a dynamoelectric machine comprising a irst conductive segment, a second conductive segment, an insulator segment positioned between said first conductive segment and `said second conductive segment, a dovetail slot positioned in said commutator at said insulator segment, the narrow portion of said dovetail slot being positioned at the surface of said commutator and being wider than and spanning said insulator segment, said dovetail slot extending over the length of the commutator adapted to be contacted by an electrical contact brush.

3. A commutator for a dynamoelectric machine comprising a cylindrical array of alternate copper commutator bars and insulator segments, each of said copper comm'utator bars being insulated from adjacent bars by an insulator segment, said insulator segments lying in radial planes that extend from the surface of said commutator toward the center of said cylindrical array, a plurality of dovetail slots positioned in said commutator, each of said dovetail slots extending radially inwardly from the surface of said commutator toward the center of said cylindrical array, a dovetail slot being positioned at each of said insulator segments, with the narrow portion of said dovetail slots spanning said insulator segment at the surface of said commutator.

References Cited by the Examiner UNITED STATES PATENTS 1,247,297 11/ 17 Liljenroth 310-236 1,614,558 1/27 Kasley 90-11 2,236,257 3/41 Borchers 310-235 2,388,006 10/45 Moeller 310-235 MILTON O. HIRSHFIELD, Primary Examiner. 

2. A COMMUTATOR FOR A DYNAMOELECTRIC MACHINE COMPRISING A FIRST CONDUCTIVE SEGMENT, A SECOND CONDUCTIVE SEGMENT, AN INSULATOR SEGMENT POSITIONED BETWEEN SAID FIRST CONDUCTIVE SEGMENT AND SAID SECOND CONDUCTIVE SEGMENT, A DOVETAIL SLOT POSITIONED IN SAID COMMUTATOR AT SAID INSULATOR SEGMENT, THE NARROW PORTION OF SAID DOVETAIL SLOT BEING POSITIONED AT THE SURFACE OF SAID COMMUTATOR AND BEING WIDER THAN AND SPANNING SAID INSULATOR SEGMENT, SAID DOVETAIL SLOT EXTENDING OVER THE LENGTH OF THE COMMUTATOR ADAPTED TO BE CONTACTED BY AN ELECTRICAL CONTACT BRUSH. 